The disclosed invention relates in general to position measurement systems and more particularly to a position measurement system that can measure absolute positions in three dimensions. It is well known that an interferometer can be used to accurately measure movement in one dimension as is utilized in applications such as the manufacture of integrated circuits. A suitable interferometer for such applications is presented in U.S. Pat. No. 3,458,259, issued to A. S. Bagley et. al. on July 29, 1969 and assigned to Hewlett-Packard Company. In this interferometer, a reflector (e.g., a cube corner) is mounted onto a movable component whose position is to be monitored. A portion of a laser beam produced by a laser head is reflected from the reflector back to the interferometer to enable measurement of the distance moved by this movable component.
Another suitable interferometer, presented in U.S. Pat. No. 3,788,746 entitled Optical Dilatometer issued to Richard R. Baldwin and Bruce J. Ruff on Jan. 29, 1974, is illustrated in FIG. 20. In that interferometer, a light source 2010 produces a first beam 2012 that is divided by a polarizing beam splitter 2060 into a reference beam 2050 and a test beam 2016. The reference beam travels along a reference path AGHF and the test beam travels along a test path ABCDEF that reflects off of a movable reflector 2015 at point B. In that particular interferometer, the test beam is also reflected off reflector 2015 at point E. At point F the reference and test beams are recombined to form an interfering output beam 2016.
The following disadvantaqes exist in such an interferometer system. First, since the reflecting mirror must always be located along the axis of the laser beam, the mirror cannot be moved freely in three dimensions, but instead is limited to 1-dimensional motion. Second, since the Doppler effect is utilized to measure distances, the distance moved by the movable component can be measured, but the absolute distance from the laser head to the movable component cannot be measured. Third, the movable component is moved only in response to control instruction (e.g., commands from a computer). Fourth, a time consuming alignment is required to ensure that the laser beam is accurately incident on the laser throughout the range of motion of the movable component.
The present invention addresses the disadvantages in the system discussed above and is suitable for making both relative and absolute measurements of position of a system component that is freely movable in three dimensions.